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Summary Table

Category	Details
Risk Factors	Neonates with intraventricular hemorrhage (prematurity), prior meningitis, prior subarachnoid hemorrhage, posterior fossa tumors (children), congenital malformations (aqueductal stenosis, Chiari, Dandy-Walker), elderly patients (NPH)
Etiology	Imbalance between CSF production and absorption or obstruction of CSF flow pathways
Patient Presentation	Infants: increasing head circumference, irritability, poor feeding. Children/Adults: progressive headache worse in the morning, nausea/vomiting, vision changes. Elderly (NPH): gait difficulty, cognitive decline, urinary incontinence
Classic Physical Exam	Infants: bulging fontanelle, separated sutures, "sunset sign" (eyes deviated downward), increased head circumference crossing percentiles. Adults: papilledema, CN VI palsy, altered consciousness. NPH: magnetic gait (feet "stuck to the floor"), frontal-type dementia, urge incontinence
Key Diagnostic Results	CT head (best initial test): ventricular dilation out of proportion to sulcal enlargement, Evans index > 0.3. MRI brain: identifies cause of obstruction. NPH: normal opening pressure on LP with clinical improvement after large-volume tap
Management	Acute with herniation risk: emergent external ventricular drain (EVD). Definitive: ventriculoperitoneal (VP) shunt. Obstructive hydrocephalus: endoscopic third ventriculostomy (ETV). NPH: VP shunt placement after positive large-volume LP trial. Temporizing: acetazolamide
Keywords	"Increasing head circumference," "bulging fontanelle," "sunset eyes," "headache worse in the morning," "wet, wacky, and wobbly," "magnetic gait," "ventriculomegaly," "aqueductal stenosis," "Evans index"

1. Pathophysiology

The brain normally produces approximately 500 mL of cerebrospinal fluid (CSF) per day through the choroid plexus, primarily located in the lateral, third, and fourth ventricles. At any given time, only about 150 mL of CSF circulates within the ventricular system and subarachnoid space. CSF flows from the lateral ventricles through the foramina of Monro into the third ventricle, then through the aqueduct of Sylvius into the fourth ventricle, exiting via the foramina of Luschka (lateral) and foramen of Magendie (midline) into the subarachnoid space. Absorption occurs at the arachnoid granulations (also called arachnoid villi), which drain into the dural venous sinuses, predominantly the superior sagittal sinus.

Hydrocephalus develops when there is a mismatch between CSF production and absorption, or when there is obstruction of the normal flow pathways. This leads to progressive ventricular dilation and increased intracranial pressure (ICP). The elevated ICP is directly responsible for the clinical symptoms: headache from stretching of pain-sensitive meningeal structures, nausea and vomiting from pressure on the area postrema in the floor of the fourth ventricle, and papilledema from transmission of elevated pressure along the optic nerve sheath. CN VI palsy is a classic "false localizing sign" because the abducens nerve has the longest intracranial course and is vulnerable to compression against the petrous bone when ICP rises diffusely.

In infants, the open fontanelles and unfused cranial sutures allow the skull to expand, which is why the classic presentation is a rapidly increasing head circumference with a bulging anterior fontanelle rather than the headache and papilledema seen in older patients. The "sunset sign" (tonic downward deviation of the eyes) results from pressure on the tectal plate of the midbrain, which houses the vertical gaze center. This is essentially a form of Parinaud syndrome caused by dorsal midbrain compression.

In Normal Pressure Hydrocephalus (NPH), the pathophysiology is distinct. The ventricles are chronically enlarged, but the CSF pressure measured on lumbar puncture is within the normal range (hence the name). The prevailing theory is that intermittent pressure waves (B-waves) cause progressive ventricular dilation over time, even though the baseline pressure normalizes between episodes. The periventricular white matter, particularly around the frontal horns, is compressed and becomes edematous. This damages the corona radiata motor fibers to the legs (which lie medially, as predicted by the motor homunculus), producing the characteristic gait apraxia. Frontal lobe dysfunction accounts for the dementia, and disruption of periventricular pathways to the sacral micturition center causes urge incontinence.

A frequently tested concept is ex vacuo ventriculomegaly, which must be distinguished from true hydrocephalus. In neurodegenerative diseases such as Alzheimer disease or frontotemporal dementia, brain parenchyma atrophies and the ventricles passively enlarge to fill the space. There is no increase in ICP, and crucially, the sulci are proportionally enlarged alongside the ventricles. This is not hydrocephalus and does not require shunting.

2. Classification and Clinical Manifestation

Classification by Mechanism

Type	Mechanism	Common Causes	Ventricular Pattern
Non-communicating (Obstructive)	Blockage within the ventricular system itself	Aqueductal stenosis, posterior fossa tumors (medulloblastoma, ependymoma, pilocytic astrocytoma), colloid cyst of the third ventricle, Chiari malformation, Dandy-Walker malformation	Ventricles proximal to the obstruction are dilated; ventricles distal are normal or small
Communicating (Non-obstructive)	Impaired CSF absorption at the arachnoid granulations (or rarely, CSF overproduction)	Post-subarachnoid hemorrhage, post-meningitis (bacterial, TB), post-intraventricular hemorrhage (premature neonates), choroid plexus papilloma (overproduction, rare)	All ventricles are diffusely dilated
Normal Pressure Hydrocephalus (NPH)	Chronic communicating hydrocephalus with intermittent pressure waves but normal baseline ICP	Idiopathic (most common), secondary to prior SAH, meningitis, or trauma	Ventriculomegaly with disproportionately small or effaced sulci; periventricular lucency
Ex Vacuo (not true hydrocephalus)	Passive ventricular enlargement due to loss of brain parenchyma	Alzheimer disease, Pick disease, Huntington disease, chronic ischemic changes, advanced HIV encephalopathy	Ventriculomegaly with proportionally enlarged sulci

Clinical Manifestation by Age Group

Age Group	Presentation	Key Exam Findings
Neonates/Infants (open fontanelle)	Increasing head circumference, irritability, poor feeding, vomiting, excessive sleepiness, failure to thrive	Bulging and tense anterior fontanelle, splayed cranial sutures, "sunset sign" (tonic downward eye deviation), scalp vein distension, increased head circumference crossing percentile lines, "cracked pot" sound on percussion (Macewen sign)
Children/Adolescents (closed sutures)	Progressive headache (worse in the morning or upon waking, improved by standing), nausea, vomiting (especially projectile), vision changes, declining school performance, personality changes	Papilledema, CN VI palsy (unilateral or bilateral), upgaze palsy, increased head circumference (if sutures not yet fully fused), hyperreflexia, gait instability
Adults	Headache, nausea, vomiting, visual obscurations, cognitive decline, lethargy progressing to obtundation	Papilledema, CN VI palsy, altered mental status, Cushing triad in late/impending herniation (hypertension, bradycardia, irregular respirations)
Elderly (NPH)	Insidious onset of gait difficulty (earliest and most prominent), followed by cognitive decline, then urinary incontinence. Classic mnemonic: "Wet, Wacky, and Wobbly"	Magnetic gait (wide-based, shuffling, feet appear "glued to the floor"), frontal-type dementia (apathy, inattention, slowed processing), urge incontinence. Notably: no papilledema (pressure is normal)

NPH: Order of Symptom Appearance and Response to Treatment

Feature	Details
First symptom to appear	Gait disturbance
Last symptom to appear	Urinary incontinence
First symptom to improve after shunting	Gait disturbance
Least likely to improve after shunting	Dementia (especially if long-standing)

This pattern is extremely testable. The gait disturbance in NPH is both the earliest manifestation and the best predictor of shunt responsiveness.

3. Diagnostic Workup

Test	Role	Key Findings
CT Head without contrast	Best initial test for all age groups	Ventriculomegaly, periventricular lucency (transependymal CSF flow), Evans index > 0.3
MRI Brain	Most accurate test for identifying cause of obstruction; best for anatomy	Defines site and nature of obstruction (tumor, aqueductal stenosis, Chiari), CSF flow studies (cine MRI) can demonstrate flow dynamics
Cranial ultrasound	Best initial test in neonates (through open fontanelle)	Ventricular dilation, intraventricular hemorrhage grading in premature infants
Large-volume lumbar puncture (30-50 mL)	Diagnostic and therapeutic trial for NPH	Clinical improvement in gait after removal of 30-50 mL of CSF (positive "tap test"); opening pressure is characteristically normal ($< 18\ cm\ H_2O$)
Lumbar puncture with opening pressure	Helps differentiate hydrocephalus types	Elevated in obstructive and communicating hydrocephalus; normal in NPH and ex vacuo
Continuous CSF pressure monitoring	Used when LP tap test is equivocal for NPH	Detection of pathological B-waves supports NPH diagnosis
Head circumference measurement	Serial monitoring in infants	Crossing upward across percentile lines is a red flag

CT Head is the best initial test in any patient presenting with signs of elevated ICP, including suspected hydrocephalus. It is fast, widely available, and reliably shows ventricular dilation. The Evans index is calculated as the ratio of the maximum width of the frontal horns to the maximum internal diameter of the skull at the same level. A value greater than 0.3 is diagnostic of ventriculomegaly. On CT, you should also look for periventricular hypodensity (transependymal flow of CSF), which indicates active hydrocephalus with elevated pressure, as opposed to stable or compensated ventriculomegaly.

A critical distinction on imaging is differentiating true hydrocephalus from ex vacuo ventriculomegaly. In true hydrocephalus, the ventricles are dilated but the cerebral sulci are effaced or small because pressure is pushing brain tissue outward against the skull. In ex vacuo change, both the ventricles and sulci are enlarged proportionally because brain tissue has been lost.

MRI Brain is ordered after CT when you need to define the cause of hydrocephalus, particularly in non-communicating (obstructive) types. MRI can identify the exact point of obstruction: aqueductal stenosis will show a dilated third ventricle and lateral ventricles with a normal-sized fourth ventricle, while a posterior fossa tumor will show a mass compressing the fourth ventricle. Cine MRI (phase-contrast MRI) can assess CSF flow dynamics at the aqueduct, which is especially useful in planning endoscopic third ventriculostomy.

In neonates, cranial ultrasound through the anterior fontanelle is the preferred first-line study because it avoids radiation and sedation. It is the standard tool for screening premature infants for intraventricular hemorrhage (IVH) and monitoring post-hemorrhagic hydrocephalus.

For NPH, the diagnostic approach centers on demonstrating that the ventriculomegaly is responsible for the symptoms and that symptoms improve with CSF drainage. The large-volume lumbar puncture ("tap test") involves removing 30 to 50 mL of CSF and then reassessing the patient's gait before and after. Improvement in gait strongly predicts a good response to VP shunt placement. Importantly, a negative tap test does not rule out NPH; some patients with NPH will not improve with a single tap but will still respond to shunting. If the tap test is equivocal, continuous lumbar CSF drainage over 48 to 72 hours or continuous ICP monitoring looking for B-waves can be pursued.

4. Management and Treatment

Clinical Scenario	Intervention	Details
Acute hydrocephalus with impending herniation	External ventricular drain (EVD)	Emergent bedside procedure; catheter placed into the lateral ventricle to drain CSF and monitor ICP. Temporizing measure, not definitive
Definitive treatment (most cases)	Ventriculoperitoneal (VP) shunt	Most commonly used permanent CSF diversion; catheter from lateral ventricle to peritoneal cavity. Valve regulates flow
Obstructive hydrocephalus (select cases)	Endoscopic third ventriculostomy (ETV)	Fenestration of the floor of the third ventricle to bypass the aqueductal obstruction; avoids shunt dependency. Best candidate: aqueductal stenosis
NPH	VP shunt (after positive tap test)	Adjustable-pressure valve preferred; gait improvement is expected first
Temporizing medical therapy	Acetazolamide (25 mg/kg/day, can be titrated up)	Carbonic anhydrase inhibitor that reduces CSF production. Used as a bridge in neonates awaiting shunt or when surgery is not immediately possible
Post-hemorrhagic hydrocephalus in neonates	Serial lumbar punctures or ventricular reservoir (Ommaya) before VP shunt	VP shunt placement is typically deferred until CSF protein is low enough to avoid shunt obstruction

Acute Management: When a patient presents with acute hydrocephalus and signs of elevated ICP (declining consciousness, Cushing triad, or imaging showing impending herniation), the next best step is emergent EVD placement. This is a bedside neurosurgical procedure that allows immediate CSF drainage and continuous ICP monitoring. It is not a permanent solution. Patients are typically positioned with the head of bed elevated to 30 degrees. The EVD is kept at a set height above the external auditory meatus to regulate drainage.

VP Shunt Placement is the definitive treatment for most forms of hydrocephalus. A proximal catheter is placed in the lateral ventricle (usually the right, through a frontal or occipital burr hole), connected to a one-way valve that opens at a set pressure threshold, and a distal catheter is tunneled subcutaneously to the peritoneal cavity where the CSF is absorbed. Alternative distal sites include the right atrium (ventriculoatrial or VA shunt) or the pleural space, but these are second-line options used when the peritoneum is not suitable (e.g., after extensive abdominal surgery or peritonitis).

VP Shunt Complications

Complication	Presentation	Management
Shunt infection (most common organism: S. epidermidis)	Fever, altered mental status, meningismus, abdominal pain (peritonitis from distal catheter)	Shunt removal, external drainage, IV antibiotics (vancomycin), then new shunt after CSF is sterile
Shunt malfunction/obstruction	Return of hydrocephalus symptoms (headache, vomiting, lethargy), shunt series showing discontinuity or catheter migration	Shunt series (X-ray from head to abdomen tracing the entire catheter), CT head showing increased ventricle size compared to baseline, surgical revision
Over-drainage	Postural headaches (worse when upright, improved when supine), subdural hematoma or hygroma from tearing of bridging veins	Adjustable valve pressure increase, bed rest, surgical evacuation if symptomatic subdural collection
Slit ventricle syndrome	Chronic over-drainage leading to very small ventricles, intermittent headaches	Valve adjustment, possible shunt revision

Endoscopic Third Ventriculostomy (ETV) is the preferred treatment for obstructive hydrocephalus caused by aqueductal stenosis or posterior fossa tumors. It creates a fenestration in the floor of the third ventricle, allowing CSF to bypass the obstruction and flow directly into the prepontine cistern. The advantage is that it avoids lifelong shunt dependency. It is most effective in patients older than 6 months with non-communicating hydrocephalus. It is not effective for communicating hydrocephalus, because the problem in communicating hydrocephalus is absorption, not obstruction.

NPH Management: After a positive large-volume LP trial demonstrating gait improvement, the patient is referred for VP shunt placement with an adjustable-pressure valve. The adjustable valve allows postoperative fine-tuning to balance adequate drainage against the risk of over-drainage and subdural hematoma. Patients should be counseled that gait is the symptom most likely to improve, while dementia, particularly if present for a long time, may not improve.

Acetazolamide reduces CSF production by inhibiting carbonic anhydrase in the choroid plexus. It is used as a temporary measure, particularly in premature neonates with post-hemorrhagic hydrocephalus who are too small or too unstable for shunt surgery. The typical starting dose is 25 mg/kg/day, which can be increased to 100 mg/kg/day. Side effects include metabolic acidosis and electrolyte disturbances. It is not a substitute for surgical intervention in progressive hydrocephalus.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Idiopathic Intracranial Hypertension (IIH, Pseudotumor Cerebri)	Headache, papilledema, CN VI palsy, elevated opening pressure on LP	Ventricles are normal or small on imaging (not dilated). IIH occurs classically in obese women of childbearing age. Hydrocephalus shows dilated ventricles
Alzheimer Disease (vs NPH)	Both present with progressive dementia in the elderly	NPH has the triad: gait disturbance (prominent and early), dementia, and incontinence. Alzheimer presents with memory loss as the dominant and earliest feature, without prominent gait disturbance. Imaging in Alzheimer shows proportional sulcal and ventricular enlargement (ex vacuo)
Parkinson Disease (vs NPH)	Both present with gait abnormality and shuffling steps in the elderly	Parkinson has a resting tremor, rigidity, and bradykinesia that respond to levodopa/carbidopa. NPH gait is wide-based and "magnetic" (feet stuck to the floor), not the narrow-based festinating gait of Parkinson. NPH does not have resting tremor
Vascular Dementia (vs NPH)	Cognitive decline, gait disturbance, and urinary incontinence can overlap	Vascular dementia has a stepwise decline with a history of strokes or vascular risk factors. Imaging shows multiple infarcts rather than ventriculomegaly out of proportion to sulcal size
Brain Tumor (vs hydrocephalus)	Both can present with headache, vomiting, papilledema, and focal neurological signs	Imaging will show a focal mass lesion as the cause. Hydrocephalus from a tumor is secondary, and the tumor itself will be visible on CT/MRI. The vignette will often describe focal deficits or seizures pointing to the mass
Meningitis (vs acute hydrocephalus)	Headache, vomiting, altered mental status, fever	Meningitis presents acutely with fever, nuchal rigidity, and photophobia. CSF analysis shows pleocytosis and abnormal protein/glucose. Hydrocephalus can develop as a complication of meningitis but is distinguished by imaging showing ventricular dilation
Congenital Macrocephaly (familial)	Large head circumference in an infant	Head circumference follows a consistent percentile curve (does not cross upward). Fontanelle is soft and flat. Development is normal. Family history reveals large head size in a parent

6. Traps and High-Yield Pearls

The single most common trap with hydrocephalus questions involves Normal Pressure Hydrocephalus. Students often miss the diagnosis because the word "normal pressure" is confusing: they assume that if ICP is normal, the patient cannot have hydrocephalus. The key is to recognize the clinical triad ("wet, wacky, and wobbly") in an elderly patient with ventriculomegaly on imaging and to know that the tap test (large-volume LP with gait reassessment) is the diagnostic maneuver.

A second major trap is the ex vacuo distractor. A vignette will present an elderly patient with dementia and imaging showing "enlarged ventricles," leading students to choose hydrocephalus. The discriminator is whether the sulci are enlarged proportionally. If sulci are widened alongside the ventricles, this is brain atrophy, not hydrocephalus, and no shunting is needed.

Another frequently tested concept is the order of symptom appearance and treatment response in NPH. Students who remember the triad but not the sequence will miss questions asking which symptom appears first (gait) or which improves most with shunting (gait). Dementia that has been present for years is less likely to improve with shunt placement, and this is often the tested nuance.

In pediatric vignettes, the trap is recognizing hydrocephalus in an infant. The vignette will describe an infant with an enlarging head, bulging fontanelle, and "downward gaze deviation" (sunset sign). Students sometimes mistake this for a seizure presentation or a craniosynostosis vignette. The key difference is that craniosynostosis presents with an abnormally shaped head (due to premature closure of sutures) but a normal or small fontanelle, whereas hydrocephalus presents with a uniformly enlarged head with a bulging, tense fontanelle and separated sutures.

Finally, shunt complications are a favorite testing topic. Students should know that the most common organism in VP shunt infection is Staphylococcus epidermidis (not S. aureus), and the management requires complete shunt removal (not just antibiotics alone). Over-drainage causing subdural hematoma is another tested complication, particularly in elderly patients whose bridging veins are already stretched due to brain atrophy.

The core competency being tested across hydrocephalus questions is the ability to integrate clinical presentation, imaging findings, and patient demographics to arrive at the correct diagnosis and then select the appropriate next step in management. Whether it is recognizing the NPH triad, distinguishing communicating from non-communicating types based on which ventricles are dilated, or knowing when to place an EVD versus a VP shunt versus an ETV, exam questions in this domain reward systematic clinical reasoning over rote memorization.

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